Block copolymers have molecular structures in which polymer subunits having chemically different structures from each other are linked by covalent bonds. Block copolymers are capable of forming periodically aligned structure such as the sphere, the cylinder or the lamella through phase separations. Sizes of domains of the structures formed by the self assemblies of block copolymers may be adjusted in a wide range, and various shapes of structures can be prepared. Therefore, they can be utilized in pattern-forming methods by lithography, various magnetic recording mediae or next generation nano devices such as metal dots, quantum dots or nano lines, high density magnetic storage mediae, and the like.